Thermal Mass Anyone?

Reader Contribution by John Kosmer
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Any net zero-energy home (one that actually produces at least as much energy annually as it consumes) needs two things. The first is the sun, providing passive solar energy.

Needing the sun for energy is why some Passive House retrofit homes leave me baffled. In his lecture at The Cooper Union in NYC, Wolfgang Feist said that southern exposure is helpful in achieving Passive House energy efficiency requirements, but not essential. Not all homes you wish to renovate have a decent southern exposure.

But here’s the problem with that position. In its simplest form, we are discussing the heat you create for your home and how well the house is able to retain that heat. In a home without southern exposure (to provide external energy from the sun for passive solar heat and/or solar electric), you rely solely on the home’s insulation and on energy created by internal heat sources (like a heating unit or your family).

So let’s say you have two homes that are super insulated to the same degree. One home has negligible external heat or electric gain and the other has south-facing windows that allow passive solar heat gain and electric solar panels, which together provide anywhere from 50% to 100% of the home’s energy needs. Obviously, the passive solar home has a much better chance of being net zero energy or more, because it’s using external energy.

The second necessity is retaining maximum heat. That is simple enough with a house that has no southern exposure: super insulate it. In a home with southern exposure there is an additional requirement. How do you store the excess energy coming into your home for when you need it? With electric solar panels, if you have net metering, it is easy. The excess energy is fed to the electric grid. The electricity you use and the excess electricity you produce is tallied up at the end of the year; you pay or receive a settlement amount.

With passive solar heat gain it is not that easy. You have to have your own place to store that heat. The concrete slab is practical for heat storage; it’s a “thermal battery”. An established principle in physics is that everything wants to even out to the same temperature. That’s why ice melts in a glass of water. The ice heats up and the water cools down until they reach the same temperature. The concrete slab works the same way. It stores heat when the slab is cooler than the air above it (during the day) and releases heat when the slab is warmer than the air above it (in the evening). There is some disagreement, however, about the optimal thickness of that slab. Daniel Chiras notes, on p. 30 in his 2002 book The Solar House, “…the thicker the mass the more heat it absorbs, up to a four-inch depth. Beyond that, however, the effectiveness of mass trails off. In fact, a six-inch mass wall is only 8% more effective than a four-inch wall.” Addressing the advantages and disadvantages of thicker verses thinner mass slabs, he acknowledges, “unfortunately, there’s not much science to back up either view.”

I live in a home with a twelve-inch concrete slab. It comes from 35-years of experience with over 300 homes that my solar engineer, Bruce Brownell, incorporated into the passive solar homes he has designed.

One of the bad raps that thick thermal slabs get is that when they are too thick, they act as a heat sink. The slab sucks heat in without ever letting heat out. I have not experienced that in my home. If the slab is super insulated, the heat has nowhere else to go but back out through the top surface where it was absorbed. Moreover, even if the 12″ slab takes longer to heat up, it will retain a more constant temperature longer and release its heat more slowly and evenly, which makes for more comfortable living. My 29 ft. by 51 ft. concrete slab is sitting on thick insulation and a humidity barrier over a gravel base. The ground temperature is 40 to 50 degrees in the central portion of the slab. It only gets cooler as you approach the perimeter of the slab because the external ground surface near the slab cools the cools the ground around the slab’s perimeter. Most of the slab is sitting on a ground surface that has a much closer heat differential (70 degrees for the slab verses 40-50 degrees for the ground) than the 0 – 32 degrees outside. This means even if there is some heat dispersion down and out through the bottom of the slab, most of the slab is dispersing heat at a much slower rate into the warmer ground.

My home has ducts embedded in the 12″ slab that carry air to exchange heat with the slab. Over the slab is a bamboo floor glued to a plywood floor that is nailed and glued onto nominal 2″ X 6″ sleepers (laid on their flat side and staggered every 2 feet.) The sleepers are glued and nailed (nails with loads shot into the slab) into the concrete slab. The wood floor acts as insulation to retard the heat from coming up from the top of the slab. It allows more of the heat to be efficiently channeled through the ducts and out the floor vents.

Photo by John Kosmer. The team is pouring 10″ of concrete over a previous 2″ pour (called the rat slab). Over the rat slab the main trunk (encased in plywood for strength) has been installed and the metal ducts from it have been laid out and bolted into the rat slab to prevent them from floating during the pour.